This invention relates to electric fuses, and more particularly, to a high voltage current limiting fuse operated at high temperatures and having a fuse element comprised of a metallic material that is highly susceptible to oxidation when exposed to such high temperatures.
High voltage fuses for interrupting a wide range of currents are well known in the art. One such high voltage fuse is described in U.S. Pat. application Ser. No. 270,032 J. G. Leach et al, filed June 3, 1981, now U.S. Pat. No. 4,357,588 and assigned to the same assignee of the present invention, and herein incorporated by reference.
It is generally a requirement of current limiting fuses that the quantity I.sup.2 t required to melt the element be kept to a low level. The quantity I.sup.2 t is meant to represent the product of the square of the current (I.sup.2) conducted by the fuse element multiplied by a duration (t). In order that this quantity be held at a low value, highly conductive materials such as silver, copper, and aluminum have commonly been used for the fuse element.
Although copper is a desired material for fuse elements its potential usage has been somewhat restricted. The copper material is primarily used for "cool running fuses". The term "cool running fuses" is meant to represent fuses whose fusible element(s) operating temperatures are relatively low such as a temperature within the range of 25.degree. to 125.degree. C. The usage of the copper material as a fuse element is severely limited in hot running fuses. "Hot running" fuses is a term commonly used to signify those fuses having fusible element(s) operating temperature which exceeds the previously mentioned temperature range of 25.degree. C. to 125.degree. C. for considerable periods of time (many hours).
The limitation of the copper material to cool running fuses is primarily created by oxidation problems. The copper is highly susceptible to oxidation when it is exposed to a high operating temperature such as 200.degree. C. The high operating temperature tends to require the use of metals such as silver, platinum, iridium, and other expensive non or low oxidizing materials. It is desired from an economic viewpoint that a relatively inexpensive material such as copper be adapted to be used in hot running high voltage fuses even though such materials are highly susceptible to oxidation at high temperatures.